FetchSandbox as an MSW Alternative
Mock Service Worker is the right tool for unit testing and frontend development. When your test cares about what happens AFTER the request, you need a stateful sandbox instead.
Where MSW shines
MSW intercepts requests at the network layer in the browser and Node, which is exactly what you want for unit tests and component tests. It is the cleanest way to stub a single response without standing up a separate process. For frontend dev work and React Testing Library tests, MSW is hard to beat.
Where MSW stops being enough
MSW handlers are stateless by default. Your handler returns a hardcoded response. When your test does a POST followed by a GET on the same resource, the GET has no idea that the POST happened — it just returns whatever you configured. That works for one request in isolation. It does not work for the actual integration flow you are trying to ship.
You can build your own state on top of MSW handlers, but at that point you are maintaining a hand-rolled fake API. That is exactly the work FetchSandbox does for you automatically.
Side-by-side
Use MSW for
- Unit tests and component tests
- Stubbing one specific response
- Frontend dev when the backend is missing
- In-process request interception (no separate server)
Use FetchSandbox for
- Integration tests that span multiple requests
- Webhook handler tests with real events
- Lifecycle state machine testing
- CI runs that need per-session isolation
They are not mutually exclusive. Most teams keep MSW for component tests and use FetchSandbox for the integration layer.